The invention relates to data systems communicating by wireless links and more particularly to adapting the size and coding of packets in order to reduce over-all delay when communicating data over the wireless links.
In wireless packet data systems such as Enhanced General Packet Radio Service (EGPRS), selective Automatic Repeat Request (ARQ) is used for error recovery over the wireless link. In an earlier analysis, Radio Link Control (RLC) performance was usually characterized in terms of the throughput, while the size of the higher layer Protocol Data Units (PDUs) or packets to be transferred was ignored.
In wireless packet data systems, such as Enhanced General Packet Radio Service (EGPRS), selective ARQ is used for error recovery over the radio link, see for example K. Balachandran, R. Ejzak, and S. Nanda, xe2x80x9cEfficient transmission of ARQ feedback for EGPRS radio link control,xe2x80x9d in IEEE Vehicular Technology Conf., May 1999. Currently, nine Modulation and Coding Schemes (MCSs) have been proposed for EGPRS with MCS-1 having the most robust coding and MCS-9 having the least robust coding.
Each higher layer packet is segmented into multiple Radio Link Control (RLC) blocks. The RLC block size and the number of RLC blocks transmitted in a 20 ms block period vary depending on the MCS chosen for transmission. For example, the RLC block sizes for MCS-1 and MCS-2 are 22 octets and 28 octets, respectively. For MCS-1 to MCS-6, a single RLC block is transmitted in a 20 ms block period on each time slot. For MCS-7 to MCS-9, two RLC blocks are transmitted in a 20 ms block period on each time slot. This state of the art is known from: A. Furuskar, S. Mazur, F. Muller and H. Olofsson, xe2x80x9cEDGE: enhanced data rates for GSM and TDMA/136 evolution,xe2x80x9d IEEE personal Communications, pp. 56-66, June 1999; ETSI GSM 03.60, xe2x80x9cDigital cellular telecommunications system (phase 2+); General Packet Radio Service (GPRS); service description: stage 2xe2x80x9d; and K. Balachandran, K. Conner, R. Ejzak, and S. Nanda, xe2x80x9cA Proposal for EGPRS Radio Link Control Using Link Adaptation and Incremental Redundancy,xe2x80x9d Bell Labs Technical Journal, vol. 4, no. 3, pp. 19-36, July-September 1999. If mobile stations are multi-slot capable, then multiple time slots (up to 8 in EGPRS) are available for use.
The RLC Block Error Rate (BLER) can be reduced significantly by using more robust codes. The result of using robust schemes, such as MCS-1, is that fewer rounds of retransmission are required to complete a transmission. However, stronger coding reduces the number of data bits that can be transmitted in one RLC block. Less robust schemes such as MCS-9 are able to pack more bits in a single RLC block, but operate at higher BLERs under typical channel conditions and may require more retransmissions to complete a transmission. For Selective ARQ, the throughput upper bound is R(1xe2x88x92Pe), where R is the transmission rate and Pe is the block error rate. This applies well to the transfer of large amounts of data (for example file transfer protocol (ftp) applications). However, for applications such as web browsing and telnet, the concern is with the delay in transferring short packets. So, for transferring short packets, the long term throughput does not quantify the quality of service. In what follows, the EGPRS framework is assumed in order to study the tradeoffs between Forward Error Correction (FEC) and ARQ for different packet sizes from a delay perspective.
There have been several studies on the delay performance of selective repeat (SR) ARQ. In M. E. Anagnostou and E. N. Protonotarios, xe2x80x9cPerformance analysis of the selective repeat ARQ protocol,xe2x80x9d IEEE Trans. on Communications, vol. 34, no. 2, pp. 127-135, February 1986, an exact and an approximate analysis on RLC block delay was derived in the single slot case. In R. Fantacci, xe2x80x9cQueueing analysis of the selective repeat automatic repeat request protocol wireless packet network,xe2x80x9d IEEE transactions on Vehicular technologies, vol. 45, no. 2, pp. 258-264, May 1996, the author analyzed the performance of the SR-ARQ in the Markov two-state channel. Another approximation can be found in J. Chang and T. Yang, xe2x80x9cEnd-to-end delay of an adaptive selective repeat ARQ protocol,xe2x80x9d IEEE Transactions on Communications, vol. 42, no. 11, pp. 2926-2928, November 1994. In all the above referenced works, only delay for a block has been calculated.
The important measure is not the time (delay) for delivering a block of data. Rather, the important measure is the time (delay) to communicate a message. To come closer to the delay for a message, it is desirable to measure the delay for in-sequence delivery of a higher layer packet, which contains multiple RLC blocks. Once a measure of the delay times of in sequence delivery of higher level packets containing multiple blocks is achieved, improvements in the method and apparatus for communicating blocks of data in the imperfect world of wireless communications can be also be achieved. It is desirable to provide improvements to the method and apparatus for communicating messages made up of data blocks via wireless communications in imperfect transmission/reception channel conditions.
Briefly stated in accordance with one aspect of the invention, the aforementioned shortcoming of the art is overcome by providing a method for reducing delay time of data delivery comprising the steps of storing at least one data packet to be transmitted over a link; transmitting a first segment of the data packet at a first modulation and coding scheme level; and transmitting a second segment of the data packet at a second modulation and coding scheme level which is more robust.
In accordance with another aspect of the invention, the aforementioned shortcoming of the art is overcome by providing an apparatus for reducing delay time of data delivery, comprising a buffer for storing at least one data packet to be transmitted over a link and a modulator and coder for modulating and coding a first segment of the data packet at a first modulation-and-coding level. The modulator and coder also modulating and coding a second segment of the data packet at a second modulation-and-coding level which is more robust than the first modulation-and-coding level. The apparatus has a transmitter for transmitting the first and second segments. By using a more robust code for the second segment, the delay time to the user is reduced.